export default causes CommonJS bundles to include .default

Adding `export default` to the main code causes CommonJS bundles to include .default and returns an error if not:
```
C:\Users\davet\Documents\sudokutest\index.js:15
console.log("Empty: ", sudoku.findAllEmpty(grid));
^

TypeError: sudoku.findAllEmpty is not a function
at Object. (C:\Users\davet\Documents\sudokutest\index.js:15:31)
at Module._compile (node:internal/modules/cjs/loader:1565:14)
at Object..js (node:internal/modules/cjs/loader:1708:10)
at Module.load (node:internal/modules/cjs/loader:1318:32)
at Function._load (node:internal/modules/cjs/loader:1128:12)
at TracingChannel.traceSync (node:diagnostics_channel:322:14)
at wrapModuleLoad (node:internal/modules/cjs/loader:219:24)
at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:170:5)
at node:internal/main/run_main_module:36:49

Node.js v22.12.0
```

With .default
Without .default

Reproduce the issue with the provided index.js CommonJS example under Node.js, then trace tsup's handling of export default in the CommonJS bundle. Done means the generated bundle exposes findAllEmpty directly, matching the no-.default example, without the reported TypeError.

Written by the indexing model from the issue text.